Combustion cycles for internal combustion engines may include a plurality of working strokes. During what is referred to as the intake stroke, the working chamber is filled with a fresh gas charge (air or air/exhaust gas). During what is referred to as the expulsion stroke the working chamber is emptied. The delimitation of the working chamber or combustion chamber with respect to the intake section or exhaust gas section of the machine is usually implemented by means of valves (in the case of two-stroke engines or else by means of what are referred to as control slits).
Typically, these valves are actuated by means of at least one camshaft. Between the valve and the camshaft there are usually also mechanical components for transmitting force, including valve play compensation (e.g., bucket tappet, toggle lever, rocker lever, push rod, hydraulic tappet, etc.). The camshaft or shafts are driven via the internal combustion engine itself. For this purpose, the camshaft or shafts is/are connected to the crankshaft by means of suitable adapters using a toothed belt or a control chain. The position of the crankshaft with respect to the position of the camshaft or shafts is defined by this connection (control times). Within this control link, actuating elements may adjust a desired phase offset between the crankshaft or shafts and camshaft or shafts.
These actuating elements are known as phase shifters for variable valve drive (VVD). For optimized operation of the internal combustion engine (with respect to emissions, consumption, power, smooth running etc.), the fresh gas charge which is taken in during the intake stroke should be known as accurately as possible. This fresh gas charge is dependent on the selected control times (inlet valve and outlet valve or control slits).
A reference internal combustion engine is measured in each of the operating states which occur (rotational speed, load, actuation of all the actuators, different valve strokes, actuation of flaps, actuation of the phase shifters for the inlet and outlet valves, exhaust gas turbocharger, compressor, etc.) and these measured values (or derivatives therefrom or model approaches which represent the behavior) are stored in the engine control unit of a corresponding series-production internal combustion engine. All the structurally identical, series-produced internal combustion engines of the same series are then operated with this generated reference data set.
A deviation of the actual relative position between the camshaft and the crankshaft from the reference position (deviation of the control times) at a series-production internal combustion engine causes the actually taken-in fresh gas charge to differ from the fresh gas charge which is determined as a reference. An angular offset of individual cams on the camshaft (compared with the reference angle determined at the reference internal combustion engine) brings about here the same fault pattern as an angular offset of the camshaft and crankshaft (compared with the reference position determined at the reference internal combustion engine). During the operation of the engine, these faults can result in negative effects with respect to emissions, consumption, power, smooth running etc.
Possible causes for the described deviations can be                angular deviation of the aimed-at relative position between the camshaft and the corresponding coupling elements such as, e.g., drive toothed wheel or drive pulley, compared to the reference position during assembly or as a result of fabrication tolerances, and        angular deviation with respect to the aimed-at phase angle offset of the individual cams on the camshaft with respect to one another during assembly or as a result of fabrication tolerances, and/or        lengthening of the control chain or of the toothed belt by means of which the camshaft and the crankshaft are coupled.        
Typically, the solution of the described problem is the detection and quantification of the deviations which occur between the reference internal combustion engine and the series-production internal combustion engine in order to be able to carry out corresponding measures for correction or compensation.
In order to counteract this problem, it has previously been attempted to minimize the fabrication tolerances as far as possible, for example the camshafts and the corresponding coupling elements are assembled with mechanical aids. In addition, the control times are measured on the respective stationary series-production internal combustion engine on the basis of valve stroke adjustment, cam contour, etc., and the internal combustion engine is adjusted during assembly.
Most currently known systems operate with a reference point system (position feedback). Here, in each case a position mark, which can be detected with a sensor, is placed at any desired point on the camshaft or a coupling element or a possibly present phase shifter etc., and, for example, on the flywheel of the crankshaft. As a result, the relative position between the crankshaft and the camshaft can be determined and deviations from the aimed at reference values can be identified. The undesired effects of these deviations can then be compensated in the control unit by adapting or correcting corresponding control variables as a function of the determined deviations.
However, only some of the tolerances which occur can be detected. For example, it is not possible to detect an angular deviation owing to a deviation of the position of the camshaft encoder wheel with respect to the camshaft or an angular deviation with respect to the aimed-at phase angle offset of the individual cams with respect to one another during assembly or as a result of fabrication tolerances (cylinder-specific deviation).
Further methods, such as evaluation of the knocking sensor signal or evaluation of the cylinder pressure signal, are also known. For example, U.S. Pat. No. 6,804,997 B1 discloses an engine control device for determining the crankshaft position and the engine phase by monitoring intake air pressure oscillations. The control device determines intake air pressure oscillations which indicate an intake air event and therefore a specific crankshaft position as well as its corresponding period of the engine cycle. The control device uses this information to determine the crankshaft rotational speed and crankshaft position to control the fuel injection and the ignition behavior of the engine.
Furthermore, document DE 10 2005 007 057 discloses a regulating method for a throttle-valve airflow to be regulated, in the intake section of an internal combustion engine, wherein pressure pulsations in the intake section, which pulsations are also influenced, inter alia, by the valve control times of the internal combustion engine, are taken into account during the regulation of the fluid flow. For this purpose, the pressure pulsations are analyzed by means of fast Fourier transformation, and the amplitude information is combined in a distortion factor which is used as an additional input variable, for example, for a multi-dimensional mathematical regulating model of the throttle-valve airflow. Specific conclusions about the valve control times of the internal combustion engine cannot be drawn by means of this method.
Document DE 35 06 114 A1 discloses a method for performing open-loop or closed-loop control of an internal combustion engine in which at least one manipulated variable of the internal combustion engine is controlled as a function of an operating variable which contains at least part of an oscillation spectrum of the internal combustion engine as information, such as for example, gas pressure signals. For this purpose the value spectrum contained in the detected operating variable is determined therefrom, as part of the oscillation spectrum, by discrete Fourier transformation and is used as a measurement spectrum and compared with a reference spectrum. That manipulated variable of the internal combustion engine which is to be controlled is then controlled as a function of the deviation between the measurement spectrum and the reference spectrum. A specific conclusion about the valve control times of the internal combustion engine cannot easily be drawn either using this method.
Document US 2009 0 312 932 A1 discloses a method for diagnosing the combustion within an internal combustion engine, wherein a combustion phase-adjustment value is generated from the crankshaft angular speed by means of a fast Fourier transformation, and this value is compared with an expected combustion phase-adjustment value, and differences between these values which are greater than a permissable combustion phase-adjustment difference are identified. A similar procedure for determining deviations between a reference engine and a series-production engine as described above is also disclosed in document US 2010 0 063 775 A1.